1. Field of the Invention
The present invention relates to an optical disk drive for rotating an optical disk and a recording and reproducing apparatus for recording and/or reproducing information by using the optical disk.
2. Description of the Related Art
In recent years, in the field of information storage, there has been much research on the optical recording system and magneto-optic recording system. The magneto-optic information recording system has the advantages that high speed recording and reproduction are possible, that various forms of memory such as playback only types, additional write types, and rewriteable types can be easily handled, that utilization as an exchangeable medium is easier in comparison with magnetic recording type disks, etc. and has a broad range of application from the industrial to the consumer sector as a system enabling realization of cheap, large sized files.
As playback only type recording media, compact disks, video disks, etc. have been spread. Mini Discs etc. have spread as rewritable recording media.
Note that, as the recording medium of the optical recording system and magneto-optic recording system, an optical disk comprised of a disk-like optical disk substrate on which a recording layer made of various types of functional films is formed is general.
Among the optical disks, a magneto-optic disk is comprised of an optical disk substrate formed with a recording layer comprised of a vertically magnetized film made of a rare earth metal-transition metal amorphous alloy such as a TbFeCo alloy as the functional film.
In order to record or reproduce information to or from a magneto-optic disk, a laser beam or other beam is irradiated from a transparent substrate side. At the recording of information, the coercive force of the vertically magnetized film is lowered at the position where the laser beam or other laser beam is focused. By applying a magnetic field to this focused position from the outside, the magnetization of the focused position is inverted to record the information.
In order to read the recorded information by the length and direction of magnetization, use is made of the Kerr effect of the polarization direction of a reflected laser beam rotating in different directions according to the direction of magnetization when a linear polarized laser beam is reflected at a magnetized film surface.
When recording and reproducing information, the optical disk is rotated around its center axis by for example a spindle motor or other motor. A center hole is formed at the center portion of the optical disk. As an example, the center hole is fitted over a rotary shaft of a turntable, the optical disk set on the turntable, and the turntable rotated by the spindle motor to rotate the optical disk.
Summarizing the problems to be solved by the invention, in the optical recording system and magneto-optic recording system, an increase of the density of the recorded information is being sought. In order to achieve this, it has been required to increase the numerical aperture (NA) of the object lens for condensing the laser beam or other light on the optical disk or shorten the wavelength λ of the light.
If increasing the numerical aperture of the object lens, coma aberration tends to occur. This coma aberration occurs when the beam supposed to vertically strike the recording surface of the optical disk strikes the optical disk at an angle and increases in proportional to the cube of the numerical aperture of the object lens and the thickness of a light transmission portion of the optical disk.
For this reason, it is possible to make the substrate of the optical disk thinner to reduce the coma aberration. Reducing the thickness of the optical disk is also effective from the viewpoint of suppressing birefringence. However, the reduction of the thickness of the substrate of the optical disk makes it difficult to secure rigidity of the optical disk.
Generally, a magneto-optic disc is enclosed in a cartridge and handled as an exchangeable medium.
The spindle motor and the optical disk are clamped inside the optical disk drive by a clamping plate made of a magnetic material arranged in the vicinity of a clamping area of the optical disk and a clamping magnet attached to the turntable.
The attraction force (clamp force) of the magnet is generated at an inner circumference side of the disk from the clamping area.
If warping occurs in a radius direction in the optical disk due to such an attraction force, the beam to be focused on the optical disk at the time of recording and reproduction of the information no longer vertically strikes the optical disk substrate. As a result, the reflected beam does not correctly return to the object lens or other receptor, but coma aberration is generated and therefore correct servo control and correct recording and reproduction of information become difficult.
Accordingly, the attraction force of the magnet must be set to a value that suppresses bending of the optical disk occurring due to attraction to not more than a predetermined value.
In a recording and reproducing apparatus recording and reproducing information by using an optical disk, however, the user sometimes operates the recording and reproducing apparatus around a vertical axis vertical to the rotational axis of the optical disk rotating by the motor. In this case, a moment (gyro moment) is generated in the optical disk. For example, when the recording and reproducing apparatus is a disk camcorder recording a video signal, the user sometimes twists the camcorder about to move the field of view of a viewfinder.
Here, an orthogonal coordinate system having an X-axis, a Y-axis, and a Z-axis is set up. A case where an optical disk of an inertia moment I is rotated about the Z-axis within an XY plane at an angular velocity Ω is assumed. When this optical disk is rotated around the Y-axis at the angular velocity ω, a gyro moment M is generated. The magnitude thereof becomesM=I×ω×Ω  (1) 
When the attraction force of the magnet is small, the optical disk is liable to detach from the top of the turntable due to the gyro moment M.
For this reason, the attraction force of the magnet must be set to a value where the optical disk does not detach from the turntable by the gyro moment generated when the recording and reproducing apparatus or the optical disk drive is used.